1. Field of the Invention
This invention relates generally to electrically and mechanically interconnecting one electronic component to another electronic component to form an electronic assembly and, in particular, to the rework of ceramic ball grid array (CBGA) and ceramic column grid array (CCGA) assemblies whereby a new chip is soldered to a printed circuit board which has been reworked by the removal of a defective chip from the board and the board became warped in the rework process.
2. Description of Related Art
Forming an electronic package assembly whereby an electrical component such as an integrated circuit chip is electrically and mechanically connected to a substrate such as a card, or board, another chip or another electronic part is well-known in the art. This technology is generally termed surface mount technology (SMT) and has gained acceptance as the preferred means of joining electronic package assemblies. in one particular application, to which this application is directed to for convenience, multilayer ceramic components, as exemplified by integrated circuit chips, are joined to printed circuit cards or boards which boards have been reworked by removing defective or unwanted chips and which boards are warped due to the rework process.
Multilayer ceramic electronic components are typically joined to printed circuit boards by soldering pads on a surface of one of the electronic components to corresponding pads on the surface of the other component. Control Collapse Chip Connection is an interconnect technology developed by IBM as an alternative to wire bonding. This technology is generally known as C4 technology or flip chip packaging. Broadly stated, an integrated circuit chip is mounted above a board and pads on the chip are electrically and mechanically connected to corresponding pads on the board by a plurality of electrical connections such as solder bumps. The integrated circuit chips may be assembled in an array such as 10.times.10 array.
In the C4 interconnect technology, a relatively small solder bump is attached to the pads on one of the components being joined typically to the chip. The electrical and mechanical interconnects are then formed by positioning the corresponding pads on the board to be joined adjacent the solder bumps on the chip and reflowing the bumps at an elevated temperature. The C4 joining process is self-aligning in that the wetting action of the solder will align the chip bump pattern to the corresponding board pads.
A myriad of solder structures have been proposed for the surface mounting of one electronic structure to another. Typical surface mount processes form the solder structures by screening solder paste on conductive, generally metallic pads exposed on the surface of the first electronic structure or substrate. A stencil printing operation is used to align the contact mask to the pads. The solder paste is reflowed in an inert atmosphere and homogenizes the pad and brings the solder into a spherical shape. The solder spheres on the substrate are then aligned to corresponding pads on the electronic structure or board to be connected thereto. After alignment, the substrate and board go through a reflow operation to melt the solder and create a solder bond between the corresponding pads on the substrate and other electronic component. The interconnection is typically in a form of a double truncated sphere and is termed a ceramic ball grid array (CBGA).
Other known surface mount technology uses different solder connecting structures. By using solder balls, for example, a more exact and somewhat greater quantity of solder can be applied than from screening. The solder balls are aligned and are held to a substrate and melted to form a solder joint on a conductive pad of the substrate. As before, the substrate with the newly joined solder balls is aligned to the board to be connected therewith and the solder balls are then reflowed to form a solder bond between the two substrates. The use of a copper ball surrounded by eutectic solder is also used as a solder joint structure for attaching a multilayer ceramic (MLC) substrate to a PC laminate where the ball serves as a standoff. Solder columns are also used to form solder interconnections as is well-known in the art and are generally termed a ceramic column grid array (CCGA).
A variety of soldering/desoldering machines are known for attachment and detachment of electrical circuit components particularly from areas of crowded printed circuit boards where the components are adjacent to other closely-spaced soldered components which are not to be disturbed. As shown in U.S. Pat. No. 5,553,768, which patent is hereby incorporated by reference, an improved process and apparatus is provided including an improved gas nozzle device which senses and regulates the soldering and desoldering of electrical circuit components to a printed circuit board. In general, a vacuum gas nozzle is used for holding an electrical component (chip) parallel relative to a printed circuit board (PCB) surface and directing hot gas downwardly through slots in the nozzle against solder bumps on the chip and corresponding solder pads on the PCB. The hot gas melts the solder to permit the attachment or detachment, as desired, of the component relative to the PCB surface. In the '768 patent, the vacuum gas nozzle has been improved by controlling parameters such as the temperature and/or flow rate of the hot gas supply in order to control the melt conditions and dwell time and optimize the soldering/desoldering operation.
The present invention is particularly directed to the reworking of ceramic ball grid array (CBGA) and ceramic column grid array (CCGA) containing chips on printed circuit boards wherein an existing printing circuit board containing one or more chips is reworked by heating the printed circuit board to a temperature sufficient to remove one or more of the chips connected to the circuit board. Temperatures of 195-220.degree. C. are typically used to melt the solder connections to remove the chip to be replaced. Typically, the glass transition temperature (Tg) of a conventional FR4 printed circuit board is about 130.degree. C. and for some high FR4 Tg materials is about 170.degree. C. Above the glass transition temperature the FR4 board material becomes soft and warpage of the board is induced at the site on the printed circuit board where the chip was removed. To rework the printed circuit board by replacing the removed chip with a new chip, the new chip will typically not conform to a severely warped site that is warped by more than 2 mils. Accordingly, if the site on the printed circuit card or board has a warpage of more than about 2 mils the board cannot be reworked or repaired.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of joining a chip or other such component to a printed circuit board particularly to a board from which a chip has been removed and which board is warped.
It is another object of the present invention to provide an apparatus for forming solder joints between a chip or other such component and a printed circuit board particularly a printed circuit board which is being reworked and which is warped due to the rework process.
A further object of the invention is to provide reworked electronic assemblies having solder interconnections with enhanced mechanical and electrical integrity and reliability.
It is yet another object of the present invention to provide a chip and printed circuit board assembly made using the method and apparatus of the invention.
Other objects and advantages of the invention will be readily apparent from the description.